Conventionally, in circuit boards used in devices such as an optical modulator, patterning of an electrode just above an optical waveguide can be performed to use a change in refractive index caused by an electric field. Such circuit boards include a surface-mount technology (SMT)-based circuit board in which an electrode of another circuit board is directly soldered onto the surface of the board to facilitate mounting of a ground electrode and the like. If such an SMT circuit board has multiple channels of electric inputs, a plurality of ground patterns is formed on the surface of the board. Each of the ground patterns is coupled to another circuit board and serves to reduce crosstalk. Therefore, a ground electrode for connection (hereinafter, referred to as a “connection part”) and a ground wiring for crosstalk reduction (hereinafter, referred to as a “non-connection part”) are mounted on the circuit board.
Patent document 1: Japanese Laid-open Patent Publication No. 2003-249711
Patent document 2: Japanese Laid-open Patent Publication No. 2007-158856
On the surface of a circuit board, a non-connection part is covered with an insulating layer, such as a solder resist, to prevent a short circuit due to solder bridging. When the non-connection part is covered with the insulating layer, a part of a connection part can also be covered to completely cover the non-connection part. In this case, depending on the formation accuracy of the insulating layer, the position of an end of the insulating layer may vary in an extending direction of the non-connection part on the side of the connection part. This causes a variation in connection part area among connection parts in the same circuit board or different circuit boards. The variation in connection part area may adversely affect the reliability (the fatigue resistance, the connection strength) or electrical characteristics of each connection part when the connection part is coupled to an electrode of another circuit board. Therefore, the variation in connection part area in the circuit board is preferably small.